Along with the usual amperage and voltage measurements usually associated with wind generators to monitor it's performance, another useful measurement you can make is the RPM of your turbines blades. You can buy a wireless cycle computer from a bike shop for about £25, then you’ve basically got a cheap RPM
meter, amongst other things.

You can fix the magnet to your propeller, (watch the balancing) and the sensor to the generators mounting bracket so that they pass each other closely without
touching.

Unfortunately the wireless facility on the cycle computer only has a maximum range of 70cm, if this is unworkable, a wired version can be used, along with the
twisted cables!

The sensor sends pulses to the bike computer wirelessly, which then displays many useful measurements including:

Back
calculating tells us that putting d = 5,305 would give a readout of the exact
RPM (but displayed as if it were km/h). Many computers won’t display more than
two digits plus the decimal, so it might make more sense to use 530.5 cm and the
readout would then show eg 35.6 for 356 RPM

Similarly
if the computer won’t accept a wheel this large we could aim for Hz (revs per
second) instead of RPM and use 5305/60 = 88.4 cm. Hertz might actually be useful
in working out the expected power out of a dynamo – I don’t know if they quote
figures based on Hz or RPM usually.

So the
two most workable numbers would be 530.5 (for RPM/10) or 88.4 (for Hz).

In order
to help with balance you could put a magnet on two opposite points, or even on
every blade. In which case simply divide the figure given by the number of
magnets used (basically the counter will see each pass as a whole revolution
when in fact it is one half, quarter or whatever). The nearer the magnets are to
the hub the more stable the arrangement will be (the linear velocity of the
magnet increase with radius, so the forces required to pull it round in a circle
increase and cause more wobble). Remember that to measure revs the radius is not
a factor